1. Field of the Invention
Embodiments disclosed herein relate generally to wellbore fluids. In particular, embodiments disclosed herein relate generally to processes for mixing wellbore fluids.
2. Background Art
When drilling or completing wells in earth formations, various fluids typically are used in the well for a variety of reasons. Common uses for well fluids include: lubrication and cooling of drill bit cutting surfaces while drilling generally or drilling-in (i.e., drilling in a targeted petroliferous formation), transportation of “cuttings” (pieces of formation dislodged by the cutting action of the teeth on a drill bit) to the surface, controlling formation fluid pressure to prevent blowouts, maintaining well stability, suspending solids in the well, minimizing fluid loss into and stabilizing the formation through which the well is being drilled, fracturing the formation in the vicinity of the well, displacing the fluid within the well with another fluid, cleaning the well, testing the well, transmitting hydraulic horsepower to the drill bit, fluid used for emplacing a packer, abandoning the well or preparing the well for abandonment, and otherwise treating the well or the formation.
In general, drilling fluids should be pumpable under pressure down through strings of drilling pipe, then through and around the drilling bit head deep in the earth, and then returned back to the earth surface through an annulus between the outside of the drill stem and the hole wall or casing. Beyond providing drilling lubrication and efficiency, and retarding wear, drilling fluids should suspend and transport solid particles to the surface for screening out and disposal. In addition, the fluids should be capable of suspending additive weighting agents (to increase specific gravity of the mud), generally finely ground barites (barium sulfate ore), and transport clay and other substances capable of adhering to and coating the borehole surface.
Drilling fluids are generally characterized as thixotropic fluid systems. That is, they exhibit low viscosity when sheared, such as when in circulation (as occurs during pumping or contact with the moving drilling bit). However, when the shearing action is halted, the fluid should be capable of suspending the solids it contains to prevent gravity separation. In addition, when the drilling fluid is under shear conditions and a free-flowing near-liquid, it must retain a sufficiently high enough viscosity to carry all unwanted particulate matter from the bottom of the well bore to the surface. The drilling fluid formulation should also allow the cuttings and other unwanted particulate material to be removed or otherwise settle out from the liquid fraction.
There is an increasing need for drilling fluids having the rheological profiles that enable these wells to be drilled more easily. Drilling fluids having tailored rheological properties ensure that cuttings are removed from the wellbore as efficiently and effectively as possible to avoid the formation of cuttings beds in the well which may cause the drill string to become stuck, among other issues. There is also the need, from a drilling fluid hydraulics perspective (equivalent circulating density), to reduce the pressures required to circulate the fluid, this helps to avoid exposing the formation to excessive forces that may fracture the formation causing the fluid, and possibly the well, to be lost. In addition, an enhanced profile is necessary to prevent settlement or sag of the weighting agent in the fluid. If this occurs, it can lead to an uneven density profile within the circulating fluid system that may result in well control (gas/fluid influx) and wellbore stability problems (caving/fractures).
To obtain the fluid characteristics required to meet these challenges, the fluid must be easy to pump, so it requires the minimum amount of pressure to force it through restrictions in the circulating fluid system, such as bit nozzles or down-hole tools. Or in other words, the fluid must have the lowest possible viscosity under high shear conditions. Conversely, in zones of the well where the area for fluid flow is large and the velocity of the fluid is slow or where there are low shear conditions, the viscosity of the fluid needs to be as high as possible in order to suspend and transport the drilled cuttings. This also applies to the periods when the fluid is left static in the hole, where both cuttings and weighting materials need to be kept suspended to prevent settlement. However, it should also be noted that the viscosity of the fluid should not continue to increase under static conditions to unacceptable levels. If this occurs, it can lead to excessive pressures when the fluid is circulated again that can fracture the formation, or alternatively it can lead to lost time if the force required to regain a fully circulating fluid system is beyond the limits of the pumps.
Depending on the particular well to be drilled, a drilling operator typically selects between a water-based drilling fluid and an oil-based or synthetic drilling fluid. Each of the water-based fluid and oil-based fluid typically include a variety of additives to create a fluid having the rheological profile necessary for a particular drilling application. For example, a variety of compounds are typically added to water- or brine-based well fluids, including viscosifiers, corrosion inhibitors, lubricants, pH control additives, surfactants, solvents, thinners, thinning agents, and/or weighting agents, among other additives. Some typical water- or brine-based well fluid viscosifying additives include clays, synthetic polymers, natural polymers and derivatives thereof such as xanthan gum and hydroxyethyl cellulose (HEC). Similarly, a variety of compounds are also typically added to a oil-based fluid including weighting agents, wetting agents, orgalnophilic clays, viscosifiers, fluid loss control agents, surfactants, dispersants, interfacial tension reducers, pH buffers, mutual solvents, thinners, thinning agents and cleaning agents.
While the preparation of drilling fluids can have a direct effect upon their performance in a well, and thus profits realized from that well, methods of drilling fluid preparation have changed little over the past several years. Typically, the mixing method still employs manual labor to empty sacks of drilling fluid components into a hopper to make an initial drilling fluid composition. However, because of agglomerates formed as a result of inadequate high shear mixing during the initial production of the drilling fluid composition, screen shakers used in a recycling process to remove drill cuttings from a fluid for recirculation into the well also filter out as much as thirty percent of the initial drilling fluid components prior to the fluid's reuse. In addition to the cost inefficiency when a drilling fluid is inadequately mixed, and thus components are aggregated and filtered from the fluid, the fluids also tend to fail in some respect in their performance downhole. Inadequate performance may result from the observations that the currently available mixing techniques hinder the ability to reach the fluids rheological capabilities. For example, it is frequently observed that drilling fluids only reach their absolute yield points after downhole circulation.
Furthermore, for drilling fluids that incorporate a polymer that is supplied in a dry form, the adequacy of the initial mixing is further compounded by the hydration of those polymers. When polymer particles are mixed with a liquid such as water, the outer portion of the polymer particles wet instantaneously on contact with the liquid, while the center remains unwetted. Also effecting the hydration is a viscous shell that is formed by the outer wetted portion of the polymer, further restricting the wetting of the inner portion of the polymer. These partially wetted or unwetted particles are known in the art as “fisheyes.” While fisheyes can be processed with mechanical mixers to a certain extent to form a homogenously wetted mixture, the mechanical Nixing not only requires energy, but also degrades the molecular bonds of the polymer and reduces the efficacy of the polymer. Thus, while many research efforts in the drilling fluid technology area focus on modifying drilling fluid formulations to obtain and optimize rheological properties and performance characteristics, the full performance capabilities of many of these fluid are not always met due to inadequate mixing techniques or molecular degradation due to mechanical mixing.
Accordingly, there exists a need for improved techniques which enable efficient and effective mixing of drilling fluids.